Problem: when pressing the zero-key I don't get the zero symbols. The euqal symbol works with Shift and so does the commbination of Shift+Mod produce the } symbol. The key is working as expected in console, but not in any X-application.
I am able to get a 0 in xterminals when I press ctrl in addition to pressing the zeo-key, but unfortunately this doesn't work in any other apps.

Hmm. So no keypress events generated by the zero-key at all? That's --- strange. Do other keys generate KeyPress and KeyRelease events in xev?

All the other keys do. The "zeor" is the only symbold not generated. The other symbols work with a little workaround. Shift and zero produces the = sign, and mod1+shift makes the } symbol.
I'll try to check the guides and come back to you. So far thank you for your efford